A semiconductor Light Emitting Diode (LED) is a solid semiconductor light emitting device, which uses a solid semiconductor chip as a light emitting material and releases surplus energy by means of recombination of charge carriers, thus leading to photon emission and directly generating light like red, yellow, blue, green, cyan, orange and purple light.
According to emitting colors, LED can be divided into two groups, monochrome LED and white light LED. In the 1980s, a super high brightness red LED was invented. The early red LED was grown on a light absorption substrate and its light emitting efficiency was 1-2 lumens/watt. Later, improvements are made to the red LED by using a transparent substrate. Among all the super high brightness red LEDs, the best model had a light emitting efficiency of about 9 lumens/watt, with its emitting wavelength about 640 nm and drive current ranging from 30 mA to 50 mA. When given a voltage of 1.5V, the best model emitted gloomy light. Afterwards, a high-efficiency red LED, a high-efficiency orange-red LED and a high-efficiency orange LED formed on a gallium phosphide (GaP) substrate were developed. And a high brightness orange-red LED, a high brightness orange LED and a high brightness yellow LED were developed. The first green LED was formed by using gallium phosphide, which had a light emitting efficiency of tens of lumens per watt and the maximum drive current of 30 mA. Afterwards, a high-efficiency green LED and a green LED emerged. The first high-brightness wide-waveband gallium nitride (GaN) blue LED was successfully developed by Nichia in the 1990s, which had an optical spectrum spanning areas of purple, blue and green, and had a peak width of 450 nm. The first high brightness silicon carbide (SiC) blue LED was successfully developed by Cree in the 1990s. The first high brightness silicon carbide blue LED had a very wide optical spectrum range, and more particularly, had a large intensity in an optical spectrum range from mid-blue to purple. The peak width of the first high brightness silicon carbide blue LED ranged from 428 nm to 430 nm and the maximum drive current was about 30 mA, generally, 10 mA was used.
FIG. 1 schematically illustrates a structure of a monochrome LED in the existing methods. Hereinafter, a blue LED is taken as an example. Referring to FIG. 1, a buffer layer 11 made of n-doped gallium nitride (n-GaN) is formed on a sapphire substrate (Al2O3) 10; a multi-quantum well active layer 12 made of indium gallium nitride (InGaN) is formed on the buffer layer 11 and a cap layer 13 made of p-doped gallium nitride (p-GaN) is formed on the multi-quantum well active layer 12. The buffer layer 11, the multi-quantum well active layer 12 and the cap layer 13 collectively form a LED die. Besides, a transparent metal contact layer 14 is formed on the cap layer 13. A positive electrode 15 is electrically connected to the cap layer 13. In order to expose the buffer layer 11, a groove 16 is formed extending through the metal contact layer 14, the cap layer 13, the multi-quantum well active layer 12 and the buffer layer 11. A negative electrode 17 is formed on the bottom of the groove 16 and is electrically connected to the buffer layer 11. Thus, a voltage can be applied to the LED die through the positive electrode 15 and the negative electrode 17 to cause the LED to emit light.
In the prior art, a sapphire substrate 10 is generally employed to form a LED. Since the material of the sapphire substrate 10 is insulative, it is necessary to form a groove 16 and expose a buffer layer 11 through a negative electrode 17. The groove 16 may reduce an effective emitting area of the LED.